That is when people at the apartments began to gather, some recording video of Booker pacing back and forth.

Thomas, the man arrested Sunday, was standing with the crowd.

Police said someone is heard on cell phone video telling Thomas that they would give him $200 to beat Booker.

Thomas is accused of then repeatedly punching Booker while calling the transgender woman derogatory names, according to the arrest warrant.

"This case is certainly disturbing," said Dallas police Lt. Vincent Weddington, commander of the crimes against persons unit. "The video shocks the conscience of anyone who looks at this video and it is something that the Dallas Police Department takes seriously."

Thomas later admitted to his role in the beating, but denied calling her derogatory names, according to his arrest affidavit.

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Police said a second person stomped on the woman's head but hasn't yet been charged.

Other people are heard hurling derogatory names at Booker, according to the arrest warrant.

Community activists gathered at the apartments on Monday and spoke directly to those who stood by and did nothing.

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"I want to say to those who live in the Royal Crest Community: Y'all are better than this," said activist Olinka Green who lives nearby. "This act of mob violence was vicious, it was uncalled for, but we must understand this is now part of our culture where people now feel like it's OK to video mob attacks, violence, danger and act of danger to those who are weaker, who cannot defend themselves."

Police said Booker's injuries sent her to the hospital, but by Sunday her family told NBC 5 that she was back home, recovering there. Her family said she suffered a broken wrist and several face fractures.

The assault happened Friday between 5 p.m. and 6 p.m. at the Royal Crest Apartments in the 3500 block of Wilhurt Avenue.

Thomas is being held in the Dallas County Jail on a charge of aggravated assault causing serious bodily injury. He is being held on $75,000 bond. It's not clear if he has obtained an attorney.

Leslie McMurray, a transgender woman who is an education and advocacy coordinator at the Resource Center in Dallas, called the attack "terrifying" and said transgender people need hate crime protection.

"It's when that message is sent time after time after time, that these people are not worth as much as these people. And transgender people should be on an equal footing as anyone else," McMurray told NBC 5.

Last November, the FBI reported that 7,175 hate crimes were committed in the United States in 2017, the most recent year for which the agency had compiled data. Of those, 1,130 were based on sexual orientation bias and 119 on gender identity bias. The data showed a 5% increase in hate crimes motivated by sexual orientation bias and a 4% decrease in hate crimes motivated by gender identity bias. Of crimes motivated by gender identity bias, 106 targeted transgender people, a 1% increase from 2016.